The present invention relates to a tire bead core and, more particularly, to a bead core that is part of a tire bead and is intended to lock the tire onto a corresponding seat of a wheel rim during various operating conditions of the tire.
As is known in the art, a conventional tire is generally formed of at least one carcass ply having edges turned up around beads, and one tread band. Specifically, a radial tire comprises a radial carcass with reinforcement wires oriented along the meridian planes of the tire, and a reinforcement belt placed between the carcass and the tread band. Strips of rubber are used to fill the enclosed area between the sides of the carcass ply and the upturned edges of the same ply.
The portion of the tire comprising the bead core and the rubber filling forms the bead of the tire, and is designed to anchor the tire to a bead seat of a corresponding mounting rim. The rim comprises a central cylindrical seat from which diverging surfaces branch axially outward from opposite sides, and terminate in vertical peripheral flanges (the rim balcony). The diverging surfaces constitute the bead seats for the tire beads.
The inner diameter of the tire beads essentially coincides with the diameter of the innermost surface of the bead core, except for a difference between the two profiles caused by the upturned plies, which form a thin layer of rubber enclosing the bead cores. The inner annular dimensions of the bead cores and of the beads are smaller than the outer diameter of the rim balcony, and are chosen so that after the tire is mounted on the rim the beads are forced onto the respective rim seats, upon which they remain in a state of mechanical tension stress.
The operations of mounting the tire onto the rim are performed according to methods well known to those skilled in the art. The operation starts by deforming the first bead of a tire into an oval configuration, so that when positioned in front of the rim with the oval aperture suitably oriented, a portion of the bead slips over the balcony of the rim. Then the rest of the bead completely slips over the rim balcony, so that the bead can then be pushed toward the bead seat. The preceding steps are then repeated for the second bead. Finally, the tire is inflated to press the beads against the internal surfaces of the rim balcony of the bead seat.
Several types of bead cores are known. For example, one design provides for a bead core formed of a rubber-coated steel wire wound in a spiral to form a first layer of coils placed side by side. Subsequent layers are superposed to the first layer and consist, as the first layer, of helicoidal windings of the same coated wire. One known construction of this type comprises four layers, each with four coils.
Another design for a bead core uses several individual wires, including a first wire wound in a spiral to form several coils arranged radially along a vertical plane, and subsequent wires similarly wound in coils along vertical planes and placed next to the first plane. The bead wire in this case is also rubberized. One particular construction of this type of bead core is known as 4xc3x974 and comprises four coils of wires and four layers of coils.
A further design uses a coiled bead core, formed by a central cable around which several wires are wound helically.
The bead core is an essentially circumferentially inestensible component formed from a single element, or from several individual annular elements such as wires, cords, and the like. The first step in the mounting of the tire over the rim therefore requires the application of considerable force to deform the bead core from its circular configuration into an oval one. In the following steps, the force is removed and the bead core reacts elastically to the deformation applied by exerting an elastic grip on the rim seat. When the tire is in operation, this elastic grip guarantees functioning of the tire by keeping the tire beads tightly attached to the rim.
One solution for easy mounting a tire bead onto a wheel rim constitutes constructing a bead core for a tire from a shape-memory material such as a Ni-Ti alloy. This solution is taught by German Patent Application DE 3829460 A1. The method described in that application calls for deforming the bead core into an oval configuration and, after insertion on the rim, submitting the bead core to a heat treatment at the crystallization temperature of the alloy, which ranges between 60xc2x0 C. and 95xc2x0 C., so that the bead core recovers its annular shape and its original dimension. This method facilitates mounting the tire and provides a tight grip on the rim after mounting, but requires specialized equipment.
One of the principal requirements that a bead core material must satisfy is that the associated tire bead be able to adequately grip the bead seats of the rim during normal operating conditions, after the bead core has been stretched for mounting on the rim and the deforming force has been removed. A second requirement consists of maintaining, and even increasing that grip force in extreme operating conditions. These conditions result, for example, during aggressive driving conditions.
Modern tire construction techniques tend to be determined more and more frequently by the driver""s tendency to use a sports car type driving style, characterized by sudden accelerations and braking, by traveling over steep stretches of road, and over roads with continuously changing curves. There also exist a frequent tendency of certain drivers to stretch the performance characteristics of their vehicles, even though those vehicles do not have high-performance features. Those drivers tend to adopt a driving style characterized by sudden accelerations and braking, with continuous changes in velocity. For simplicity, that style of driving is referred to here as xe2x80x9cirregular-type driving.xe2x80x9d
The aforementioned irregular type driving affects the bead cores, since variations in acceleration can induce strong variations in torque transmitted to the tires, with the risk that the tire beads will slip on the rim seats if the bead cores do not adequately grip against the rim seats. There is also now a requirement for tires capable of traveling several tens of kilometers while deflated. Current practical requirements expect the deflated tire to run for fifty or more kilometers, for example, to a suitable repair station where the flat tire can be repaired or replaced. The requirement of adequate grip of the tire bead to the rim seat when the tire is in a deflated condition is complicated by the absence of air pressure in the tire. This pressure normally is used for pushing the bead against the flange surface of the rim balcony, thus keeping the tire securely attached to the rim.
There are known devices designed to allow a tire to run while deflated. Some of these known devices use protuberances extending from the toe of the beads which are introduced into corresponding holes on the rim, to resist the inward displacement of the beads from the mounted position which occurs when air pressure is lost. Other solutions are based on providing axially along the rim suitable separation devices designed to impede inward movement of the beads when air pressure no longer holds them against the bead seats.
Additional solutions include the use of devices comprising a container located on the rim, and containing a lubricating liquid. When the tire deflates, the approach of the bead portion to the tread causes the cover of the container to open, and the lubricant flows out to seal the perforation in the tire.
None of the conventional solutions allow simple mounting of the tire to a rim at ambient temperature, and also allow operation of the tire in the deflated condition.
The applicant realized that the tire could be made to operate safely in regular or irregular conditions, including when flat, if the bead cores were made with properties that guarantee a practically constant or increasing grip of the tire bead onto the bead seats of the rim in any condition. This solution avoids the need for tire-mounting operations that require temperatures considerably above ambient temperature, as well as the assembly of sophisticated deflation-resisting devices placed on the rim inside the tire, or modifications of the conventional profile of the beads by forming protuberances that have to be inserted into holes in the rim.
Applicant began by noting that modern tire accessories are specifically selected by the manufacturers to achieve significant braking performance improvements. These accessories include rims made of metal alloys and brakes made of materials which increase the braking forces generated, such as carbon brakes acting on disks connected to alloy rims.
It was then noticed that the thermal heating of the above-mentioned materials forming the rim and brakes assembly provided an available source of heat. The heat flows to the zones of the tire adjacent the brake, such as the beads and the bead cores. Furthermore, when the tire operates deflated, the elastomeric material of the tire is further heated by the relative sliding of the folded parts of the sidewall rubbing against each other, with a consequent increased transmission of heat toward the beads and bead cores.
Thus, the increased heating and transmission of heat toward the bead cores was identified to be a common result of irregular type driving conditions and of operating with a deflated tire. Applicant then thought that such a common result could be harnessed to make the tire operate safely in all conditions, while still retaining the ability to easy mount the tire on the rim at ambient temperature and with conventional tools.
A material for a bead core was sought that had two major characteristics. First, the material had to be elastically deformable at ambient temperature, so that the bead core could be stretched over the rim balcony and then locked elastically on the bead seat to provide a gripping force on the rim. Second, the material had to respond to temperature increases by developing an increased force of contraction greater than the elastic grip force generated at ambient temperature. This increased force should take effect once a predetermined temperature is reached during operation.
A first aspect of the invention is thus a bead core for a tire for locking to a seat of a wheel rim having a plurality of annular reinforcement elements with an inner transverse dimension smaller than a maximum diameter of the wheel rim, and being sufficiently deformable at ambient temperature for mounting the tire onto the wheel rim when a force is applied to the bead core. Another aspect of the bead core comprises using annular reinforcement elements with tightening means that respond to temperature increases by increasing the tightening force on the wheel rim, with at least one of the tightening means being made of a shape-memory alloy. The tightening force begins to increase when a predetermined temperature is reached. This second aspect is provided by tightening means which are formed by including in the plurality of reinforcement elements forming the bead core at least one element made of a metal alloy having shape-memory properties. This element having shape-memory properties may not tightly grip the rim after mounting at ambient temperature, but tightens on the rim as the tire temperature increases. Preferably, the predetermined temperature at which the tightening means begin to respond to temperature increases by increasing the gripping force is greater than 80xc2x0 C., but below a maximum temperature reached when the tire operates deflated.
The reinforcement elements forming the bead core are thus made by first and second reinforcement elements. The first reinforcement elements are designed to provide the grip of the bead core onto the rim at ambient temperature, while the second elements form the tightening means sensitive to temperature increases. The first reinforcement elements are made from metal or textile wires such as steel or aramid fiber, or from metal cords. The tightening means of the second reinforcing elements are made from wires or metal cords, and are preferably arranged in the innermost radial position of the bead core, closest to the tire axis of rotation. Preferably, the second tightening means are made of materials of a family of alloys of NiTi, NiTiX (X=Fe, Cu, Nb), CuTiAl, CuAlNi, CuAlBe, FeMnSi-based alloys, and FeNiCo-based alloys.
A further aspect of the invention is a tire comprising a toroidal carcass, a tread band, and a belt structure placed between the tread band and the carcass;
a pair of beads located at the two edges of the carcass, and a pair of bead cores located inside the beads. The beads are designed for securing the tire onto the bead seats of a wheel rim. Each bead core comprises a plurality of reinforcing elements that extend annularly, and have an internal transverse dimension nearly the same as the diameter of the bead seat of the rim.
The tire is further characterized by having a bead core which comprises tightening means sensitive to temperature increases, providing a contraction force that begins to increase when a predetermined temperature is reached by the tire bead. At least one of the plurality of annular reinforcement elements is made of a shape memory metal alloy material. The bead core of the tire includes first and second reinforcement elements as earlier defined. For example, the first reinforcement elements can have an overall Young""s modulus between 50,000 MPa and 205,000 MPa and a breaking load between 1,500 MPa and 4,000 MPa. The tightening means sensitive to temperature increases of the second reinforcement elements can have an overall Young""s modulus between 50,000 MPa and 120,000 MPa, and a breaking load between 800 MPa and 1,200 MPa.
In one preferred embodiment, the tire includes tightening means suitable for developing, at a temperature between 95xc2x0 C. and 110xc2x0 C., a locking force between tire bead and rim that is 20% to 30% greater than the locking force exerted at ambient temperature.
The present invention will be better understood with the aid of the following description and attached figures, provided solely by way of a non-limiting example. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.